1. Field of the Invention
The present invention relates to a voltage controlled oscillator used for a phase locked loop (PLL) circuit etc.
2. Description of the Related Art
A voltage controlled oscillator is an essential circuit for making a PLL.
In particular, in the recent various wireless, wire, and fiber optic types of communications systems, a voltage controlled oscillator capable of operating at a high speed with a small jitter and reduced phase noise is required in a PLL circuit for generating a frequency standard for modulation and demodulation and for extracting a clock signal.
Note that jitter and phase noise are just different ways of viewing the same fluctuation, that is, in a time domain or in a frequency domain, and mean substantially the same physical phenomenon.
General purpose voltage controlled oscillators can be divided roughly into an LC type and a CR type.
The LC type is set in oscillation frequency by an inductor (L) and a capacitor (C).
The CR type is set in oscillation frequency by a capacitor (C) and resistor (R) or an electric current.
The LC type has small jitter, however, the range of variation of the oscillation frequency is narrow and the inductor (L) has to be provided outside the IC chip.
The CR type offers a wide range of variation of the oscillation frequency and enables a good linearity to be obtained in the control characteristic of the frequency characteristic with respect to the control voltage.
Also, it is easy to integrate all of the necessary elements on an IC chip. However, there is a disadvantage that the jitter performance is inferior to that of the LC type.
Therefore, each type can be used in accordance with the specific application taking into consideration the respective advantages and disadvantages. The LC type is used mostly in wireless communications systems handling narrow band signals, while the CR type is often used in wire or fiber optic communications systems.
There are two leading types of voltage controlled oscillators of the CR type: an emitter-coupled multi-vibrator type and a ring oscillator type. These circuits of the related art generally have the following disadvantages. Improvement has been desired.
They cause interference in other circuits via a parasitic capacitance of the capacitor (C) with the IC substrate. PA1 When other circuits send signals to the IC substrate and the potential of the signals fluctuate, it suffers from interference. PA1 Due to the above two reasons, when two voltage controlled oscillators (VCO) exist on an IC chip, they interfere with each other. PA1 When the range of variation of frequency control is extremely wide, the control linearity deteriorates.
The above disadvantages will be explained in more detail below.
FIG. 12 is a circuit diagram of a first example of an emitter-coupled multi-vibrator type voltage controlled oscillator of the related art.
This voltage controlled oscillator comprises npn-type transistors Q1 to Q4, resistors R1 and R2, a capacitor (integrated capacitor) C1, diodes D1 and D2, and current sources I1 to I4.
In the voltage controlled oscillator of FIG. 12, triangular waves having a peak-to-peak (p-p) value of about 2.multidot.VF (VF=forward voltage drop of a pn junction) are generated at the two terminals of the integrated capacitor C1 due to the action of the clamp diodes D1 and D2.
The slope of the triangular waves is determined by the integrated capacitor C1 and a control current IO, thus the oscillation frequency becomes proportional to the control current IO. The forward drop voltage of the clamp diodes D1 and D2 varies, strictly speaking, in accordance with the control current 10.
When the product of the resistors R1 and R2 and the control current IO is less than VF/2, the clamp diodes D1 and D2 lose their clamping function. Accordingly, the circuit no longer functions as a voltage controlled oscillator.
Therefore, the range of variation of the frequency of this circuit is not very wide and the control linearity is not that good.
FIG. 13 is a circuit diagram of a second example of an emitter-coupled multi-vibrator type voltage controlled oscillator of the related art improved over the circuit in FIG. 12.
This voltage controlled oscillator comprises npn-type transistors Q11 to Q18, resistors R11 to R14, a capacitor (integrated capacitor) C11, diodes D11 and D12. and current sources I11 to I17.
In the voltage controlled oscillator in FIG. 13, the amplitude of the triangular waves at the terminal of the integrated capacitor C11 can be made stabler basically by inserting a differential stage comprised of the transistors Q17 and Q18. comparing this circuit with the first example of the related art shown in FIG. 12, both the range of variation of the frequency and the control linearity are greatly improved.
However, there is a problem when the control current varies for example by a factor of 10. This is because although the amplitude between collectors of the transistors Q11 and Q12 changes according to the control current IO, if the amount of change is too great, it can no longer be covered by just the amplitude limiting action of the transistors Q17 and Q18.
Next, an explanation will be made of the other leading means of a CR type voltage controlled oscillator, that is, a ring oscillator type voltage controlled oscillator, of the related art.
FIGS. 14A to 14D are views for explaining the principle of the ring oscillator type voltage controlled oscillator.
This circuit is structured to connect n stages of logic buffer circuits and to return signals in order to give negative feedback from the output to input.
The oscillation frequency fosc becomes 1/(2.multidot.tpd). Here, tpd is a propagation delay time of the logic buffer circuit.
In the circuit in FIG. 14A, three logic buffer circuits G21 to G23 having differential input/outputs are used. In the case of differential input/outputs, there is no limit on the number n of the stages. In the case of a single end inverter (inversion circuit), the number n has to be an odd number to obtain negative feedback. Generally three or more stages are used. The reason will be explained later.
FIG. 15 is a circuit diagram of a first example of a ring oscillator type voltage controlled oscillator of the related art.
This circuit comprises npn-type transistors Q31 to Q42, resistors R31 to R36, and current sources 131 to I39.
In the voltage controlled oscillator in FIG. 15, three differential buffers having emitter followers are connected in series. The current sources I31 to I33 of the current Ix are for the differential buffers, while the current sources I34 to I39 of the current Iy are for the emitter followers. When using this circuit as a voltage controlled oscillator, however, the current Ix is fixed and the current Iy is changed to change the propagation delay time.
The circuit shown in FIG. 15 oscillates at a considerably high frequency and has a considerably poor control linearity as shown in FIG. 16.
The reason for the poor linearity is that the propagation delay time tpd becomes longer by the amount of reduction of the control current Iy, but even when increasing it, the delay of the differential stage becomes dominant. In the end, the propagation delay time tpd does not become short much at all.
FIG. 17 is a circuit diagram of a second example of a ring oscillator type voltage controlled oscillator improved over the related art shown in FIG. 15.
This circuit is structured with the capacitors C31 to C33 inserted between the emitters of the transistors Q33 and Q34, Q37 and Q38, and Q41 and Q42 constituting the differential emitter follower.
The circuit of FIG. 17 can be used from a relatively low frequency. The propagation delay time tpd is determined by the capacitors C31 to C33 and the control current Iy, and the control linearity is improved.
The above explanation was made concerning the operation and specific examples of the leading types of RC type voltage controlled oscillators, that is, the emitter-coupled multi-vibrator type and the ring oscillator type.
Next, the problems in the related art will be explained.
First, the biggest problem is that interference is caused in and received from other circuits via the parasitic effects of the integrated capacitors.
FIG. 18 shows a metal insulator semiconductor (MIS) capacitor as a typical structure of a capacitor in an IC.
This MIS capacitor is formed by sandwiching a dielectric comprised of a nitride film 5 by an N+ silicon layer 4 and an aluminum (Al) integration layer 6.
However, a parasitic junction capacitance is formed with a p-type substrate 1 on the silicon side of the capacitor. Namely, the circuit becomes the equivalent circuit as shown in FIG. 19.
Recently, a capacitor of a structure with a dielectric called an MIM capacitor sandwiched by metal on at the two electrodes has been developed. In so far as the substrate is not an insulating substrate such as GaAs, generation of some kind of parasitic capacitance, though large or small in value, cannot be avoided. In a capacitor of the MIS structure, there is a parasitic capacitance of about 1/10 of the capacitance C.
The circuits of the related art explained with reference to FIGS. 12, 13, and 17 perform complete differential operations. When there is a parasitic capacitance in the integrated capacitors used by such circuits, the balance of the circuit operation is lost.
Taking as an example the voltage controlled oscillator of the emitter-coupled multi-vibrator type of FIG. 12, the integrated capacitor C1 has been constructed, as shown in FIG. 20, by connecting two MIS capacitors by antiparallel connection so as to make the parasitic capacitance enter symmetrically and prevent the loss of the balanced operation.
Next, the waveforms of the capacitor terminals A and B will be considered. When the voltages of the capacitor terminals are VA and VB, the waveforms become as shown in FIGS. 21A to 21C.
The waveforms of the terminals A and B of the integrated capacitor C31 in the case of the ring oscillator type circuit shown in FIG. 17 are shown in FIGS. 22A to 22C.
At the integrated capacitor terminals, triangular waves are generated in the case of the emitter-coupled multi-vibrator type, while trapezoidal waves are generated in the case of the ring oscillator type. When the waveforms are inverted, the voltage of the terminals of the integrated capacitor rises sharply.
Due to this rise, a considerably large spike current flows into the IC substrate via the parasitic capacitance. This is shown in FIG. 23.
This spike current makes the potential in the IC substrate fluctuate.
The spike current also flows to the Vcc terminal and the GND terminal and causes interference in other circuits via the limited impedance.
Further, this parasitic capacitance makes the voltage controlled oscillator susceptible to interference from other circuits.
Assume that there is another voltage controlled oscillator and that the first voltage controlled oscillator injects a spike current to the IC substrate. Due to this, the potential of the substrate immediately under the second voltage controlled oscillator fluctuate to certain extent. As a result, the current is injected to the circuit via the parasitic capacitance as shown in FIG. 24.
This causes a major problem especially when the oscillation frequencies of the two voltage controlled oscillators are close. An example of this can be seen for example in the PLLs on the write side and the read side of a hard disk storage device. The write side PLL determines the timing for the circuit to perform a write operation.
As opposed to this, the read side PLL extracts a clock from the signals read from the disk. Therefore, the PLL has to follow changes in the rotational speed of the disk and irregular rotation. Accordingly, there is some fluctuation in the number of clocks.
When the two PLLs interfere with each other, their characteristics deteriorate. This phenomenon is not limited to hard disk storage devices and is widely known in the wireless communications field. When the frequencies of two PLL are close, such interference arises easily.
The reason for such interference will be explained with reference to FIGS. 25A and 25B.
When the outputs of two voltage controlled oscillators approach each other, the outputs become the same in phase or opposite in phase in some sections. How often this is repeated is determined by the difference of the two frequencies. Namely, the closer the two frequencies, the longer the period where the outputs become the same or opposite phase.
When there is interference between the two voltage controlled oscillators, the same interference is received over a long period. Accordingly, the oscillation cycle becomes unstable.
Since the phase is an integral of each cycle, even if the fluctuation per cycle is the same, the smaller the frequency difference, the greater the apparent effect in terms of jitter or phase noise.
As explained above, the voltage controlled oscillators of the emitter-coupled multi-vibration type and the ring oscillator type have the disadvantage that interference arises between circuits via the parasitic capacitance of the integrated capacitors. Improvement has been desired.
This is caused because the integrated capacitor is used in a balanced state. A circuit used with one end grounded would not suffered from such a problem.
There are other types of voltage controlled oscillators with one end grounded used for low frequencies.
However, such circuits cannot be used or cannot provide sufficient characteristics in relatively high frequency (several MHz or more) applications where emitter-coupled multi-vibrator type and ring oscillator type voltage controlled oscillators excel.
Also, while these circuits can handle single-digit multiple ranges of variation of frequencies of the single-digit order, they had difficulty handling double-digit multiple ranges.
In the emitter-coupled multi-vibrator type shown in FIGS. 12 and 13, the operation mode of the circuit changes if the control current changes too much.
The ring oscillator type shown in FIG. 15 inherently has an extremely poor control linearity.
The ring counter type shown in FIG. 17 has a considerably good linearity, but also runs into problems when the control current Iy changed too much.
On the high frequency side where the control current Iy is large, the propagation delay time of the differential buffer itself appears and the oscillation frequency becomes saturated.
Conversely, on the low frequency side where the control current Iy is small, the base current of the differential buffer is no longer negligible compared with the control current Iy and again the control linearity is deteriorated.